What defines the lines on plans and diagrams indicating visible areas from audience seating?

The correct answer, sightlines, refers to the visual lines from audience seating that indicate what parts of the stage are visible to those seated in different locations. In theatrical design and staging, sightlines are crucial because they determine how the audience perceives the performance and what they can see based on their position in the theater. Understanding sightlines helps directors and designers create a staging plan that ensures the audience can fully engage with the performance without obstructions.

The other options, while related to theatre, do not pertain specifically to the definition of lines indicating visible areas from audience seating. The proscenium arch is the frame that typically separates the audience from the stage, crucial in traditional theater but not directly tied to visibility lines. The thrust stage refers to a type of staging that extends into the audience on three sides, affecting sightlines but not directly defining them. Lastly, the stage door keeper is a role related to stage management, involving overseeing entrances and exits, and does not connect to the concept of sightlines.